The present invention relates to a method and a device for producing preforms with a special geometry, which is enlarged in comparison with the cavity in the injection mould.
In the production of plastic bottles, it is usual to firstly produce a so-called preform, which in a subsequent step is inflated into a finished plastic bottle by a blow moulding process.
In the preform production, conventionally, firstly a plastic material (e.g. plastic granulate) is melted and is introduced into the cavities of a closed injection mould. Such an injection mould generally has a plurality of identical cavities which are also designated as impressions.
In order to form the preform geometry according to FIG. 1, the impressions comprise in addition to a so-called neck region, a cavity recess, into which a core projects. The core defines the inner space of the preform, the cavity recess together with the neck region the outer contour of the preform—together, they therefore establish the geometry of the preform.
After the injecting of the polymer melt into the multiple cavities, a first cooling process is carried out, in order to achieve a sufficient rigidity for the preforms, which makes an opening of the injection mould possible. After the opening of the injection mould, the produced batch of preforms is conventionally removed from the mould with a removal gripper, wherein for this in known methods each preform is transferred into an associated, mostly cooled removal sleeve of a removal gripper.
After the removal of the preforms from the injection mould, it is known to transfer the preforms by means of a transfer gripper out from the removal gripper into a further post-cooling device. After a sufficient cooling of the preforms in this post-cooling device, they are then either conveyed directly to a blow moulding installation or are packed into a carton for storage and transportation.
For a high degree of efficiency of the installation, it is important to keep the cycle time as short as possible, in order to be able to produce as many preforms as possible. However, conflicting with a further shortening of the cycle time is that fact that a considerable amount of heat must be respectively dissipated from the preforms, in order to bring them effectively to an acceptable temperature level for packing.
This takes place on the one hand already in the injection mould itself, where a very aggressive cooling is provided both via the core and also via the cavity device. The removal sleeves of the removal gripper are also generally (water) cooled.
In addition, under the brand name “Calitec” of the company Netstal, a method has been introduced, in which the inner space of the preforms, which are situated in the removal gripper, are placed under pressure, so that they position themselves with their outer wall intimately against the wall of the removal gripper and a particularly good cooling and also a recalibration is ensured. For this purpose, the transfer pins of the transfer gripper have devices for sealing the inner space of the preforms and for introducing a fluid, for example of air.
In the sequence described above, it is usual to generate a preform with a design, as is illustrated in FIG. 1. The type of design generally results from the fact that on the removal of the preforms from the injection mould, on the one hand the core must be drawn out from the inner space of the preform, and on the other hand the preform itself must be removed from the cavity. These two procedures would not be able to be directly and readily carried out in a preform geometry as is illustrated in FIG. 2. At least a core constructed in one piece could generally not be drawn out from the inner space.
On the other hand, preform geometries as are shown in FIGS. 2, 3 and 8 have some advantages, which will be explained further below.